The proper translation of messenger RNA (mRNA) into protein on the ribosome requires the presence of a number of protein factors at both the initiation and elongation steps. A large body of information has been accumulated on the functional role of these factors but little is known about their precise molecular interactions with the other components of the protein synthetic machinery, especially the ribosome. These interactions are of paramount importance since they provide the cell with a mechanism for regulating the extent and fidelity of protein biosynthesis. Photochemical crosslinking of either unmodified or photoaffinity probe modified factor is a useful approach for examining the ribosome binding sites of initiation and elongation factors. It is direct and the reagent generated during irradiation is highly reactive and non-specific. Photoreactive groups will be incorporated into initiation and elongation factors at specific amino acid residues. The factors, either unmodified or carrying a photoaffinity probe, will be bound non-covalently to ribosomes and subsequently the factor-ribosome complexes will be irradiated, generating a reactive intermediate which should insert into either an RNA or protein component of the ribosome, The sites of attachment will be identified in terms of a specific protein or ribosomal RNA base sequence. These studies will be carried out with the various initiation and elongation factors alone, and in the presence of the other components of the protein synthesis machinery (mRNA, tRNA, other factors, GTP). These experiments are expected to elucidate the specific ribosomal components at the binding sites for the various protein factors. They will provide considerable information about the nature of factor-ribosome interactions and the possible conformational transitions which the ribosome undergoes during the different stages of protein synthesis.